Iron gall ink - Chemistry
Elmer Eusman (1998)
By mixing tannin with iron sulfate, a water soluble ferrous tannate complex is formed. Because of its solubility, the ink is able to penetrate the paper surface, making it difficult to erase. When exposed to oxygen a ferric tannate pigment is formed. This complex is not water-soluble, contributing to its indelibility as a writing ink.
In general, the color of a freshly made iron gall ink solution is rather pale. The oxygen present in water reacts to produce a light gray color in the initial ink. When applied to paper, the ink does not turn black for a couple of seconds, and preceding marks were somewhat illegible due to the initial paleness of the ink. One method to improve visibility of the ink during application is to expose the ink to air for a longer period of time before use. However, because this produces a precipitate, solid particles collect at the bottom of the holder. This is the inherent irony of iron gall ink: Making the ink more visible initially results in poorer writing properties.
There are several methods to increase the visibility of the ink before use or to counter the tendency of precipitation. One can add colorants, such as logwood (red), indigo (blue) or aniline dyes (blue or violet) to impart a stronger color to the ink. To counter precipitation, the ink can be made more acidic with vinegar or even hydrochloric or sulfuric acid. In an acidic environment, hydrogen ions in gallic acid are not easily replaced by iron (II), and the low pH inhibits the oxidation process responsible for the formation of ferric tannate.
Another option is to add a binder, such as gum Arabic to the ink mixture. The gum works as a suspension agent keeping, the ink particles from settling out in solution. Generally, the gum only has a minor function as a binder. Since a newly made ink penetrated the paper sufficiently, an extra binding agent was generally not necessary. In an older ink much of the pigment had been formed. These particles can not penetrate the paper as deeply and largely remain on the surface. In this case an extra bonding agent would be helpful to keep the ink adhered to the surface.
Gallic acid (IMAGE: 3,4,5-Trihydroxy-benzoic acid) is created by the hydrolysis of gallotannic acid. Gallotannic acid (IMAGE: penta-gallolylglucose) consists of a glucose molecule with 5 gallic acid groups or di-gallic acid (IMAGE) groups in any combination. Galls contain a high concentration of gallotannic acid but less of gallic acid. To increase the gallic acid concentration, the gallotannic acid can be hydrolyzed during preparation.of the ink. This can be achieved best by boiling the gallotannic acid in an acidic environment or by letting the gallotannic acid solution ferment. Enzymes released by the mold react with the glucose and hydrolyze the gallic acid in the process.
The iron(II) ions of the vitriol react with both gallic acid and gallotannic acid. The precise chemical structure of the iron gall ink pigment has recently been investigated by two researchers (C.H. Wunderlich and C. Krekel) and each has developed different theories on the structure of the colored compound.
Wunderlich was able to form black crystals using Fe(III)Cl3 and gallic acid in a gel of sodium silicate. He suggests that the iron reacts with the three hydroxyl groups of gallic acid and with the carboxyl group, creating a three dimensional structure. The color formation in this ferric tannate complex is due to shifting electron pairs in the benzene ring and oxygen/iron bonds. Using Mossbauer and XANES spectroscopy he was able to determine an oxidation state (Fe 3+). Wunderlich created these molecules using Fe(III) ions. However, when iron gall ink is produced using iron sulfate, the ferrous tannate complex is formed with Fe(II).
Krekel has researched pigment formation using different metals under varying circumstances. He was able to create a black pigment which he examined using different analysis techniques (mass spectrometry, infrared spectrometry and Mossbauer spectrometry). His study suggests that the black pigment, instead of a iron gallic acid complex, is an iron pyrogallol complex. His research will be published in the proceedings of the 1997 meeting of specialists in Ludwigsburg. An abstract of his results will be presented here.